Root-Knot Nematode Survey for Increased Awareness of a Threat to Urban Gardens in Missouri

Published: February 1, 2009

Root knot nematode (Meloidogyne) is a microscopic worm that can infect vegetable roots causing serious symptoms and producing galls or “knots” on the roots. These galls impede the flow of water and nutrients to the plant, causing slow growth, yellowing, wilting easily in the heat of summer, stunting, and reduced yields. Since this potential pest has a very wide host range of over 2000 plants, simple garden rotation is ineffective. Root-knot can reproduce on most lawn and garden plants, and it is very easily spread by transferring tiny amounts of infested soil. Eradication of this nematode is almost impossible outside of normal periods of extreme cold and deep soil freezing. Warmer than normal winters have permitted the root-knot nematode to survive in areas where this was not a problem in the past. Control and management of this nematode is very difficult: chemical control is not labeled for gardens, and control via cultural practices is complicated and time consuming.

The objective of this survey was to screen urban gardens of Missouri for root-knot nematode, and to determine the incidence and severity of the problem. The goal was to collect 100 soil samples in gardens from around the state, with the help of Extension Specialists, and to process them for nematodes, looking specifically for the root-knot nematode.

After late June, the soil samples were to be taken from around garden plants that were not particularly thrifty. If no symptoms were present, then one soil plug was to be taken from many different individual plants to make the detection of root-knot more likely (in case only part of the garden was infested). The soil samples were then sent to the Extension Nematology Lab for processing. Each sample was thoroughly mixed and a sub-sample of 100 cm³ was processed and examined for the presence of root-knot.

RESULTS

A total of 81 soil samples from gardens around the state were received for this survey, and 30% of them were found to be positive for root-knot nematode. These gardens can be classified into two groups:

Family or private gardens: there were 38 of these samples and 13% were positive for root-knot.

Community gardens (demonstration, public and commercial gardens were included here): there were 43 of these samples and 44% were positive for root-knot.

From the survey findings, it can be concluded that gardens with more traffic in them are more likely to be infested with root-knot nematode. Since the only way that root-knot can be introduced into a clean garden is by soil or transplants, it makes sense that the more people in a garden, the more likely this nematode will be introduced. Since a tablespoon of infested soil can contain thousands of root-knot nematodes, anything from garden tillers and hand tools to muddy boots and tires to even improperly composted material can introduce this pest. Family gardens are perhaps less likely to have had root-knot introduced since there is likely less sharing of garden equipment and transplants than in community gardens.

Below is the breakdown of community gardens by geographic area:

Springfield community gardens: 2 of 3 gardens were positive, or 67%

Jefferson City community gardens: 2 of 11 were positive, or 18%

St. Louis area community gardens: 3 of 5 were positive, or 60%

Columbia community gardens: 6 of 8 were positive, or 75%

Kansas City area community gardens: 4 of 8 were positive, or 50%

St. Joseph community gardens: 2 of 7 were positive, or 29%

It is difficult to determine how severe the infestation of root-knot is in these Missouri gardens because of the “shot gun” approach to sampling--one plug of soil from many plants in order to enhance detection of the root-knot nematode. Infested soil around a seriously infected plant would be diluted by other soil plugs from around non-infected plants. The samples ranged from the low of 15 to a high of 3000 nematodes per 100 cm³ of soil. In the community gardens 23% of the samples were above the “50 nematodes per 100 cm³ of soil” economic threshold, and 16% were above 500 nematodes per 100 cm³, which is quite high.

Another way to look at this data is to divide the state by latitude. Root-knot nematode has been common in the southern part of the state for many years. This is demonstrated with the two family gardens in the Bootheel region (Scott and Mississippi counties) which both had symptoms and very high numbers (1350 and 1725 nematodes per 100 cm³). Likewise, the two community gardens in Springfield (Greene County) were badly infested (1905 and 2895 nematodes per 100 cm³). Yet, moving slightly north of Springfield, the two family gardens in Lamar (Barton county) and five family gardens in Marble Hill (Bollinger county) all were negative for root-knot. Drawing a line across the state at about 30 miles south of I-70, we enter an area that has historically not had problems with root-knot nematode. Yet, in this survey, of the 67 samples taken in this area, 30% were positive for root-knot, and even as farnorth as St. Joseph (29% positive). As interesting as this data might be, it must be remembered that 81 soil samples for an entire state is very minimal for making generalizations.

Things to Be Learned

We know that the root-knot nematode has moved north into gardens in parts of the state that didn’t traditionally have it.

The root-knot nematode can only come into a garden by movement of soil or transplants. Great care must be taken in garden hygiene:

a. Borrowed garden implements must be free of all soil (clean in 10% bleach water).

b. Care should be taken not to bring soil or mud on boots or wheels from another garden.

c. All transplants should be purchased at a reputable dealer; transplant “gifts” should be suspect.

d. Compost from a common pile (E.g. community garden) may not be properly or completely composted and may still contain viable root-knot nematode eggs. If you are not sure of the compost sources, buy compost from a reputable dealer.

e. Fill dirt or topsoil moved into the garden may have root-knot in it.

An easy and inexpensive way to determine if your garden has root-knot nematode is to do a simple check at the end of the growing season when cleaning up the garden. While pulling up the old plants, such as tomato and peppers, look at the roots. They should be smooth. If they have bumps or “knots” along the roots, then there is a good chance that root-knot is present. A soil sample can be sent to the University of Missouri Plant Nematology Lab for confirmation. (Room 23, Mumford Hall, Columbia, MO 65211; $20 fee)

If you have root-knot in your garden, management strategies can be found at: http://ppp.missouri.edu/newsletters/MEG/archives/v14n4/a4.pdf.